Remain in UK extension for family members

[rbsjp]

For family members arriving in UK to join a settled person initial visa is granted for 33 months. What are the residency conditions for extension? How long can the spouse/children be away from UK, so that further extension is not a problem? Also if the child who was under 18 initially but turns more than 18 in these 33 months how does he/she apply for extension?

[secret.simon]

FLR(M) - form and guidance.

Child's visa will be in-line with that of the less-privileged parent.